Those people are wrong. Nothing in the Constitution (which, since 2023, governs the succession to the throne) supports their interpretation.
You can read the Constitution (in French) on Legilux, the official portal of Luxembourg legislation.
legilux.public.lu
If you prefer,
@Somebody posted an unofficial translation of the Constitution's succession law here:
If at some point it becomes obviously clear that Amalia will be the future grand duchess, I could see the grand duke (whether it be the current one or Guillaume) granting her and her younger brother the titles of prince(ss) of Luxembourg; for now, they are 'just' prince(ss) of Nassau. If...
www.theroyalforums.com
Article 56.
(1)The office of Head of State is hereditary in the direct descendants of His Royal Highness Adolphe, Grand Duke of Luxembourg, Duke of Nassau, in order of primogeniture and representation. Only children born of a marriage have the right to succeed.
(2)The person entitled to succeed may renounce it. This waiver takes the form of a written act which is irrevocable and whose effects apply only to the author.
When exceptional circumstances so require, the Chamber of Deputies may exclude one or more persons from the order of succession by a law adopted by a qualified majority.
(3) In the absence of a successor, the Chamber of Deputies shall meet no later than thirty days of the death or abdication of the Grand Duke with a view to appointing a new Head of State. The decision shall be adopted by qualified majority.
(4) The abdication of the Grand Duke requires the form of a written act which is irrevocable.
Article 132.
The provisions of Article 56 shall for the first time apply to the descendants of His Royal Highness Henri, Grand Duke of Luxembourg, Duke of Nassau.